The quadrupole ion trap has been developed and, used in practice, as a mass spectrometer since the mass selective instability mode was invented several decades ago. This technique is described in U.S. Pat. No. 4,540,884. Later, in a series of the US patents such as those numbered U.S. Pat. Nos. 4,736,101, 4,749,860, 4,882,484, methods of MS and MS/MS employing resonance ejection of ions from the ion trap have been disclosed. Based on these methods, commercial ion trap mass spectrometer instruments have been manufactured and widely used The principle of operation of these instruments can be summarised by the following operational steps: Simultaneously trap the ions with a predefined mass range by applying a trapping RF voltage to the ion trap; apply supplementary AC voltage between the two end cap electrodes to cause resonance ejection of unwanted ions and again, use the supplementary AC voltage to activiate the remaining precursor ions to cause their collisional dissociation and produce product ions; and finally, scan one parameter of the trapping RF voltage or supplementary AC voltage to cause resonance ejection of ions sequentially in the order of their mass-to-charge ratios. Thus, by measuring the ejected ion current a mass spectrum can be obtained.
As the technology has developed, performance has been improved by adding high order multipole electric field components, in particular the octapole component to the quadrupole electric field. Technically, this was achieved by stretching the distance between the end caps of the ion trap or by decreasing the asymtotic cone angle of the hyperboloid geometry. These are permanent, structural changes that give rise to non-linear resonance of ion motion; so, these ion traps are also called non-linear ion traps. However, while the non-linear resonance which is caused by the high order multipole components brings about certain improvements in performance, such as good mass resolution at fast scan, it also introduces many problems. A quadrupole ion trap with significant high order multipole components cannot work in the mass-selective storage mode as is usual in the case of a quadrupole mass filter, because the non-linear resonance line which runs through the apex region of the well known (a-q) stability diagram causes ion loss. Furthermore, the non-linear ion trap cannot provide high resolution for precursor ion selection when the resonance ejection method is used.
U.S. Pat. No. 5,468,958 (Franzen and Wang) discloses a method for dividing each end cap electrode into component parts to allow the high order multipole part of the field to be selectively switched on or off. It is claimed that this kind of ion trap is able to store ions selectively with good resolution, and scan out the stored ions with good resolution as well. In practice, however, there is no easy way to implement such a device because both RF switching and precise tuning of coupling parameters are difficult to achieve. Also, no account is taken of the problem of field distortion near the end cap apertures.
Recent studies by G. Cooks published in Analytical Chemistry Vol. 72 No. 13, 2667, demonstrates that the end cap apertures where ions enter and exit the ion trap are the principal source of distortion in the quadrupole field. Such distortion causes chemical shift and delayed ejection which leads to poor resolution of mass analysis. Adding in a high order multipole field, as is done in some commercial instruments, can avoid the adverse effects of the aperture, giving improved analytical performance, but at the same time, introduces the afore-mentioned problems associated with high order multipole fields. The present inventors have discovered that by reducing field distortion in the vicinity of the aperture of an end cap electrode high mass resolution can be achieved without a significant high order multipole field. With an adjustable small high order field near the aperture there could be the opportunity to obtain even better results.
It is an object of the present invention to at least alleviate the afore-mentioned problems.